Firstly, I think the question should be rephrased more broadly. Not just what applications, but what functionality do you want to see and etc.

Secondly, did Jolla consult the questions in the community list? I think they did, and that list already mentions tons of preferred applications and functionality. So let me repeat some key things:

* XMPP / Jingle client with OTR and ZRTP support. If Jolla can sponsor such development in Telepathy - that would be ideal. Right now Telepathy lacks such support (OTR and ZRTP). Nokia used to be the sponsor for Telepathy development (through Collabora?) but they stopped, since they abandoned Linux development. Right now secure and feature rich mobile IM clients are lacking. Jitsi is catching up, but it's written in Java and isn't the best candidate for Sailfish. Having a native client would be much better.

See:
https://bugs.freedesktop.org/show_bug.cgi?id=16891
https://bugs.freedesktop.org/show_bug.cgi?id=29904

* Support for LDAP address books in the e-mail client.